A transmission of this nature is known, for example from European Patent Publication EP-A-0,451,887. In this known transmission, the transmission of torque in accordance with the preamble of Claim 1 is realized by means of hydraulic pressure, which is applied via the driven or secondary pulley, with the aid of a so-called "master-slave" control mechanism. The "master-slave" control mechanism is based on the fact that the pinching force applied via a pulley, Arcing to the wedge-like contact of the endless element or the belt in a pulley, causes reaction forces which, as a result of displacement of the belt, result in a corresponding pinching force in the bearing of the belt in the other pulley.
Continuously variable transmissions of the present type have the current drawback that the transmission is susceptible to slip if the radius over which the transmission element is in contact with a pulley is small. Therefore, similar transmissions are known in which the pinching force is constantly applied alternately as a function of the transmission ratio during operation, to that pulley in which the radius of contact with the belt is smaller. This type of control has the characteristic feature that the hydraulics have to be very accurately controlled and that the alternations during operation may occur very frequently, depending on the conditions in traffic. Applying pinching force using the control system of the transmission to which the invention relates, namely exclusively, at least in principle, from the driven, alternatively denoted, secondary pulley, avoids such alternations and, inter alia, has the advantages that the means for applying the pinching force, in this case the hydraulics and the control mechanism for the hydraulics, can be kept relatively simple. In the standard case of applying pinching force by means of hydraulic pressure, pump pressure applied can be applied to the secondary pulley without the intervention of hydraulic control elements, and it is possible for the control mechanism to comprise relatively simple hydraulic control elements which have been optimized for their particular function.
In the known transmission, an electronic control system is used to determine the pinching force which is to be applied to the secondary pulley in order to realize a slip-free transmission of a torque to be transmitted, in the primary pulley as well. In the known transmission, the magnitude of the torque to be transmitted is determined, inter alia, on the basis of the throttle position detected and the vehicle speed detected. The pinching force determined in this way is kept as low as possible, in order to prevent energy losses while nevertheless taking into account a safety margin.
Tests and practical experience on commercially available vehicles have shown that such a control for a variable speed gearbox functions as expected and correctly under rotating conditions. Nevertheless, undesirable traces of apparently short-lived slip may be found on contact surfaces of the drive pulley and the belt in the known transmission. The present invention is based on the problem of establishing a cause for such traces of slip. The invention aims to arrive at a control system adapted so as to take into account the cause established, while maintaining the energy-efficient nature of the present control mechanism. According to the invention, this object is achieved by means of the measure according to the characterizing portion of Claim 1.